C & C Effective Wind Area?? 1

[nfisteve]

I am working on a small project for a roofing company where they need wind load calculations and a data extrapolation for the fasteners that will attach a roofing membrane to a wood roof deck. The NOA for the membrane they will use gives a fastener spacing pattern and the allowable pressure that goes along with the given fastener spacing. Because the building is in a coastal high wind area, it is expected that the design wind pressures in the field area of the roof will be within the NOA limits, but the perimeter and corner pressures will be too high, thus the need to extrapolate and come up with a denser fastener spacing for these areas.

So, here's the question...
When calculating the design wind pressures for the corners and the perimeter, how do I determine the Effective Wind Area for the calcs? The ASCE 7-10 definition for membrane roof systems (pg 507) says "the effective wind area is the area of the board or membrane secured by a single fastener", but my goal in all of this is to determine the area that needs to be secured by a single fastener! Seems like a circular reference to me and it doesn't make sense.

Most engineers that I have spoken to tell me that they have always used the smallest effective wind area of 10 SF, but no one can seem to tell me why. If this is the number that everyone uses, then why would the wind pressure tables even bother listing pressures for effective wind areas of 20 SF, 50 SF, and 100 SF?

Any help here would be greatly appreciated, Thanks!

 

[dukengnr79]

I'm not sure if this answers your question or not, but I'll take a stab. The reason they list effective wind areas of 20, 50, 100, etc -- the larger the supported area, the lower the probability that that entire surface will see the larger wind gusts simultaneously for whatever the component and cladding sustained gust requirements are. A smaller effective area has a higher probability of seeing the larger wind gust over that entire supported surface.

Roof components like purlins and rafters and individual truss members might be supporting these larger areas, so the code cuts them a little bit of a break and reduces the force of the component and cladding wind pressure.

Not sure if that helped or not!

 

[Ron]

You use the tributary area of the fastener to calculate the loads. If your fasteners are spaced 16" on center with a support spacing of 24", then the tributary area of one fastener is 16"x24" or 2.67 square feet. So if you have a corner zone uplift of 90 psf, then your fastener load would be 240 lbf. Using a tension factor of safety of 4, your fastener would have to resist a 960 lbf load. That's a lot for a single fastener, so you would decrease the spacing in order to meet the published or tested resistance of the fastener.

 

[Jerehmy]

people use 10SF for fasteners because what kind of fastener will have a trib area larger than 10SF. all EPDM roof membrane with mechanical fasteners have had less than 10 SF trib area.

 

[nfisteve]

Thanks for the quick responses guys, all helpful information. I do understand how the fastener tributary area works, what I am confused about is what pressures to use at the beginning of the design process.

Let me lay out a hypothetical example, maybe that will help:
...Suppose we are putting a tpo membrane roof system on a building that is 40' wide and 100' long, has a mean height of 14', and has a flat roof. This gives me an "a" dimension for the corners and perimeter of a = 4'.

My first step is to calculate design pressures and to then determine the fastener spacing for the roof membrane, based on the NOA for the specific membrane. The first thing I need is the design pressures for zone 1, 2, & 3 (field, perimeter, & corner), so how do I determine this? Does the size of the membrane sheets make a difference? Is the effective wind area for the corner 16 SF (4'x4')?

I can't imagine that a tributary area for fasteners (from an NOA) is what I would use for this initial design pressure calc, since the fastener spacing is what I am trying to determine.

Thanks!

 

[Jerehmy]

I guarantee that no matter what roof system you use, your fasteners will not have a trib area greater than 10SF. So use 10SF to determine fastener pressure. Also, roof membranes, including TPO, have minimum fastener spacing determined by the manufacturer. Check their literature to see their recommended fastener spacing. I'm betting its not greater than 10SF trib width.